1. Field of the Invention
This invention relates generally to the cannulation of veins and arteries under the guidance of ultrasound.
2. State of the Art
Insertion of catheters into central veins or arteries can be a difficult task because the vein or artery may be located deep within the body or may otherwise be difficult to access in a particular patient. Multiple attempts at penetration may result in extreme discomfort to the patient and loss of valuable time during emergency situations. Furthermore, central veins and arteries are often in close proximity to each other. While attempting to access the internal jugular vein, for example, the carotid artery may instead be punctured, resulting in severe complications or even mortality due to consequent blood loss due to the high pressure of the blood flowing in the artery.
To prevent complications during catheterization, it is known that ultrasonic instruments can be used to determine the location and direction of the vessel to be penetrated. Various approaches use a Doppler-only technique with no imaging. One such technique transmits ultrasonic waves via a transducer from the skin surface to the vessel. Due to the flow of blood in the vessel, or the pulsation of the vascular walls, the ultrasonic wave undergoes a Doppler shift effect which causes the reflected signal to be at a frequency different from the transmitted signal. The frequency difference between the transmitted and received signals is then converted to an electrical signal, amplified and sent to an audio speaker. The frequency of the tone emitted from the speaker increases as the frequency difference becomes greater, indicating the approximate location of the vessel. Improvements to this technique place either the transmitting transducer, receiving transducer, or both transmitting and receiving transducers within a hollow needle so that the audio signal becomes louder as the needle is turned towards a vessel within the patient""s body. While such applications are helpful in guiding the needle towards the general location of vessels, the obtainable accuracy is obviously limited. Other limitations of this technology include difficulty distinguishing veins from nearby arteries, difficulty determining when the vessel has been penetrated, and difficulty implementing the known Seldinger technique.
Other conventional approaches to identification of vessel location and direction use two-dimensional ultrasound imaging to either mark the vessel location on the skin before attempting to access the vessel using the known Seldinger technique or view the vessel as the needle tip advances towards it. See British Journal of Anaesthesia, 822-6 (1999). However, it would be desirable to improve ultrasound imaging techniques for the cannulation of blood vessels to make the use of such technology less cumbersome and more accurate.
The present invention uses ultrasound techniques in an improved method and apparatus for cannulation of blood vessels. In contrast to conventional approaches, the present invention provides a clinician with the ability to manipulate the needle during insertion with both hands while observing the progress of the needle toward and into the desired target vessel in substantially real time.
The apparatus of the invention comprises a sensor assembly including two ultrasonic, linear transducer arrays, each comprising a plurality of active imaging transducer elements, the arrays being oriented perpendicularly to each other to form a xe2x80x9cTxe2x80x9d configuration and carried by a housing. The 90xc2x0 relative orientation of the array axes provides the ability to quickly and easily image blood vessels in both the longitudinal and transverse planes as a needle with attached catheter is guided towards a target vessel. One advantage of the present invention is that the needle operator may accurately orient the needle with respect to the target vessel and may, as desired, monitor the needle at all times as it passes through the anterior wall of the vessel. Thus, this technique and apparatus may eliminate the need to insert the first, or seeker, needle used in the Seldinger technique and greatly increase the accuracy over Doppler-only techniques where the needle operator is guided solely by an audible tone. Again, it is notable that the clinician employing the present invention is enabled to manipulate the needle during insertion with both hands while simultaneously observing the progress of the needle toward and into the desired target vessel.
In other embodiments of the present invention, the sensor assembly may be used in combination with a protective sheath having a frame element associated therewith and a cover configured to encompass the sensor assembly and bearing graphics to provide means, in cooperation with the frame element, for orienting the sensor assembly and securing the sensor assembly to the patient""s body in a desired orientation.
In still another embodiment of the invention, the sensor assembly may include a housing configured to include two laterally extending protrusions or xe2x80x9cwingsxe2x80x9d proximate the lower edges of two opposing side walls, the wings each carrying a magnet thereon. This embodiment of the sensor assembly may be employed in combination with a reference location element in the form of a dielectric (such as a polymer) film or tape bearing an adhesive on one side thereof for attachment to the skin of a patient over the general location of the blood vessel to be cannulated, the tape including two laterally spaced shims of a magnetically responsive metal or polymer. The lateral spacing of the shims approximates that of the magnets, but the shims are somewhat larger than the magnets to permit the sensor assembly to be moved about by the clinician over a limited area of the patient""s body with respect to the film to precisely locate the sensor assembly. The magnets, in turn, permit such movement but exhibit magnetic fields robust enough to maintain the sensor assembly in place when it is released by the clinician.
In still a further embodiment of the invention, the housing of the sensor assembly may be configured for use with a reference location element in the form of an elongated ribbon having an adhesive coating at each end thereof, the ribbon being adhered to the skin of the patient. The ribbon extends through a slot in the sensor assembly housing, which has associated therewith at least one resilient gripping element which may be manipulated by the clinician to release tension on the ribbon to enable sliding of the sensor assembly therealong as well as limited rotation thereof with respect to the ribbon to precisely locate the sensor assembly. When a desired location of the sensor assembly is reached, then the at least one resilient gripping element is released and the sensor assembly is fixed in place.
In further embodiments of the present invention, the sensor assembly further includes at least one ultrasonic Doppler transducer element used to transmit and receive a single ultrasonic beam at an angle relative to the imaging transducer array in the longitudinal plane. The addition of the Doppler transducer element or elements provides directional blood flow and blood velocity information with regard to the target vessel and others nearby and thus improves the ability to distinguish veins from arteries. The directional information from the Doppler transducer element or elements may be converted to a color mark with one distinct color indicating blood flow in one direction and another distinct color indicating blood flow in the opposite direction. For example, when the sensor housing is appropriately aligned on the body with respect to cover markings depicting blood flow toward and away from the heart, blood flow toward the heart may be indicated with the color blue and blood flow away from the heart may be indicated with the color red. Thus, when the single color scan line is overlaid on top of a grayscale longitudinal image of a possible target vessel on a monitor screen, a blue mark on the color screen will indicate a vein and a red mark will indicate an artery. While an array of Doppler elements may also be used to provide a full-color image, a single Doppler beam reduces the complexity and cost of providing desired directional flow information.
In still another embodiment, the Doppler transducer element or elements carried by the sensor housing may be configured to transmit and receive xe2x80x9cchirpedxe2x80x9d ultrasound pulses to obtain Doppler information at discrete depths within the body. A pulse is chirped if its carrier frequency changes with time. This frequency modulation, or frequency sweeping, causes the Fourier spectrum of the chirped pulse to broaden. Thus, a digital signal processor may be used to analyze the reflected signal via a Fast Fourier Transform (xe2x80x9cFFTxe2x80x9d) algorithm to separate distances or depths of various features within the body. The phase change between transmitted and received signals is used to determine speed and direction of flow in the blood vessels.
In yet another embodiment, two pulsed Doppler elements may be used for determining speed and direction of flow in the blood vessels. In this embodiment, the two pulsed Doppler elements each comprise a group of active imaging transducer elements included in one of the linear ultrasonic transducer arrays, specifically the array hereinafter termed a xe2x80x9clongitudinalxe2x80x9d array, which is to be positioned in use over the vessels to be detected and substantially parallel thereto. The two pulsed Doppler elements, each comprising a contiguous group of active imaging transducer elements, are mutually spaced from each other along the length of the array and are each angled at the same but relatively opposing angle to a perpendicular to the plane of the array of which they are a part. The two pulsed Doppler elements each transmit and receive ultrasonic signals, by which blood flow direction and velocity may be determined.
Yet another aspect of the present invention comprises a protective sheath into which the sensor assembly may be inserted, a packaging configuration therefor and a method of use thereof. The protective sheath comprises an elongated tubular thin polymer film element, closed at one end and open at the other. The protective sheath may be tapered so as to be of larger diameter or transverse dimension at the open end than at the closed end thereof. The open end of the protective sheath is folded back over the rest of the protective sheath so that a portion comprising about one-half of the protective sheath is inside-out, or everted, and extends over the remaining portion thereof. The end of the now-everted protective sheath now open and defining a bore extending to the closed end of the sheath (the original or first open end of the protective sheath now lying adjacent and surrounding the original closed end due to eversion) is rolled outwardly back upon itself toward the closed end until only a small xe2x80x9cpouchxe2x80x9d or xe2x80x9cfootxe2x80x9d of a size suitable for receiving the sensor assembly remains, the doubled and rolled polymer film forming a toroidal shape defining a mouth of the pouch or foot. At that juncture, the skirt of material defining the now-everted original or first open end of the protective sheath is folded back over the outside of the toroidal shape of rolled polymer film. In use, the inventive protective sheath may be placed in a tray of a kit including other sterile, disposable elements of the present invention with its mouth defined by the skirt and toroidal shape of rolled polymer film facing upward. In use, the sensor assembly (which is not sterile) may be placed into the pouch or foot through the mouth and the folded-back skirt of the protective sheath grasped and pulled proximally along the cable extending to the sensor assembly to maintain sterility of the exterior of the protective sheath while encompassing the nonsterile sensor assembly and associated cable therein for use. Tabs of another material may be secured to the skirt to facilitate visual identification and grasping of the skirt.
Methods of vessel identification, a system incorporating the sensor housing of the present invention and a kit of disposable sterile components are also encompassed by the present invention.